


Simultaneous Equations

by emeraldsword



Category: Teen Wolf (TV)
Genre: Anal, Cunnilingus, Double Penetration, Fellatio, Het, Multi, Threesome, Threesome - F/M/M
Language: English
Status: Completed
Published: 2013-01-27
Updated: 2013-01-27
Packaged: 2017-11-27 03:15:53
Rating: Explicit
Warnings: No Archive Warnings Apply
Chapters: 3
Words: 6,198
Publisher: archiveofourown.org
Story URL: https://archiveofourown.org/works/657434
Author URL: https://archiveofourown.org/users/emeraldsword/pseuds/emeraldsword
Summary: <blockquote class="userstuff">
              <p>Set around the beginning of Season 2. When Stiles walks in on Erica and Derek having sex, Erica gets an idea. Erica/Derek, Erica/Derek/Stiles. </p>
<p>I accidentally wrote choose your own adventure porn...</p>
            </blockquote>





	1. In which Erica gets an idea and convinces Stiles and Derek to play along

**Notes for the Chapter:**

> Thank you so much to Deltachild for the beta! All remaining mistakes are my own.

 

Erica is a big fan of this werewolf thing. Well, who wouldn't be? Going from scaredy sixteen-year-old epileptic virgin to _femme fatale_ has been a kick, and so, obviously, has sleeping with Derek Hale. She's not under the delusion he's serious about her, even if the fact that he picked her sends a secret little thrill through her. No, she's pretty sure he loves playing the hot older guy with the sexy schoolgirl hanging off that deliciously muscled arm as much as she likes playing the sexy schoolgirl with the dangerous older man. And not just for the shocked looks on Stiles and Scott's faces when he picks her up from school. He is…for want of a better word, amused by her, interested and fond in a way that he's not with Isaac or Boyd. He seeks her out when she doesn't seek him, and while she needs him to like her, she's pretty sure she doesn't need him to be in love with her. Which is for the best, really.

                The idea had come to her three days ago now. If it worked out, this was going to be the best afternoon of her life.

**

                She had gone over to the Hale house after class, partly to report that Stiles and Scott were acting kind of weird, and partly to get laid. The information was received with a blank stare and a shrug, but the fucking was proceeding very nicely indeed, thank you very much, when there was a noise outside. Both froze automatically, Erica's legs around Derek's waist, nails digging into his shoulders. Derek inhaled deeply and Erica followed suit, trying to catch the scent of the intruder over the stench of soot and sex.

                "Stiles," Derek said in a low voice, twitching into her involuntarily at the same time as she'd inadvertently clenched around him. For a moment they stayed looking at each other, locked in place and listening as Stiles pushed open the front door and came in shouting, then Erica gave Derek a wicked, wicked grin and flipped them so Derek was on his back, knees bent with her straddling him. She's pretty sure the move only worked because he'd been distracted (vulnerable), he certainly growled loud enough for Stiles's feet to pause on the stairs and for his voice to take on a newly anxious tone.

                "Derek?"

                Derek had stared up at her with a 'what the fuck?' expression on his face, an expression that she thought was showing just a tiny bit of 'okay then.'

                Erica grinned again and dragged his hands to her hips, leaning forward with a flip of her head so she could kiss him without her hair blocking the view from the doorway. He kissed back enthusiastically and Erica had known she'd won.

                Derek's fingers were digging into her ass, his arms resting alongside her thighs and moving her faster than she'd move herself when the door swung open.

                Erica broke the kiss and leaned back against Derek's thighs (and she really must make him let her do this again because this is an awesome position), smirking as Stiles gaped at them. His eyes raked over them, unable to take in what he was seeing. She hadn't stopped moving her hips, keeping up a steady roll that had Stiles's eyes drawn to where she and Derek are joined.

                "Oh God!" he gasped before thundering away down the stairs. Erica and Derek both inhaled sharply, catching the sharp odours of shock and lust. Derek flipped her onto her back again and started thrusting in earnest. She'd come with a yell, the scent of Stiles in her nostrils, and Derek followed almost immediately with a snarl.

~

They'd gone downstairs hand in hand. Stiles hadn't left; he'd been pacing agitatedly in what used to be the kitchen. Derek had rolled his eyes at her, but his grip on her hand had tightened.

"Ever hear of knocking?" he'd said and Stiles had gone from awkward to indignant in less time than it took to blink.

"Oh, excuse me for not being able to tell the difference between 'bad things are happening' growls and 'sexy time' growls! I called your name like a million times, what more did you want me to do?"

Erica snorted.

"Did you come here for something important?" she'd asked. Stiles didn't seem able to look at her, he'd directed his answer to the wall behind her head, babbling about Scott, Jackson and lacrosse as if anyone gave a damn about Scott or Jackson or lacrosse. He couldn't look at Derek either, and Derek had taken great delight in dropping her hand and getting right up in his face. Stiles had been red-faced and spluttering and had almost tripped over his feet in his hurry to be out of there. Judging from the smell, Erica reckons he'd been at least half-hard for the whole conversation.

Derek had looked at her and raised an eyebrow. She'd raised one right back and he'd actually blushed. And that was when she'd had the idea.

**

Today, she finally goes up to Stiles in the hallway. He's with Scott, and she guesses from the way Scott smells that Stiles didn't tell Scott what he walked in on and that Scott is putting Stiles's more-extreme-than-usual incoherence down to a newly discovered crush.

"Hey Stiles," she says. "I'm kind of struggling with the math. I just don't _get_ simultaneous equations. You wanna come over to mine this afternoon, to study?"

Stiles's eyes almost bug out of his head and he starts spluttering. Scott gives him a totally unsubtle elbow to the ribs.

"He'll be there," he says, and Erica grins.

"See you at four," she says and walks…well, sashays… away.

~

                When Erica lets Stiles into her house, she's wearing a skirt that comes down to her knees (though it'll ride up more than a little when she sits down) and a top that, while it shows off her figure nicely, is a little less blatant than she's worn for a while. Stiles visibly relaxes when he sees her, though he doesn't lessen his hold on the book bag he's holding protectively in front of him.

                "You want something to drink?" she asks, holding the door open for him to come in. Instead of following her, he says:

                "If this is some bizarre thing you're doing to get back at me for…" he trails off, blushing furiously and waving a hand vaguely, "because that absolutely wasn't my fault!" Erica cocks her head slightly, and then she realises that the reason she can smell Scott is not because Stiles spent all day with him but because he's actually here, lurking around the corner and making sure Stiles actually goes in. So she smiles sweetly.

                "Don't be stupid," she says. "You're good at math and I'm terrible, that's all."

                "Lydia's great at math," Stiles says mutinously, stepping cautiously inside. Erica lets out a bark of laughter.

                "I've never spoken to Lydia in my life," she says, and Stiles blinks.

                "What, really, never?" he says, and that seems to have broken the ice because he heads past her without flinching, drops his book bag by the stairs and follows her into the kitchen, babbling something about Lydia. Erica doesn't bother to listen to the actual words, because Stiles's crush on Lydia is way old news.

                She fixes them juice and a plate of cookies, because why not, and then takes him up to her bedroom. Her sheets are clean, the condoms and lube are in her dresser drawers out of sight and her math book is open on the desk.

                It's not a total lie. She could use a little help, but not half as much as she led Stiles to believe. He's not stupid; he knows what it means when her arm touches his and he glances down and sees how much her skirt has ridden up. He looks at her and swallows, and she tries not to look too threatening. Then he moves slightly, so his thigh is pressed up against hers and continues with his explanation with only a slight tremble in his voice. Erica swallows too, feeling the pulse of excitement start in her groin.

                She slides a hand onto his thigh, feeling the muscles tense under her hand, and he stops talking mid-word, staring at her in disbelief. She slides her hand up his thigh, eyes on his face the whole time, making it quite clear where this is going.

                "Um," he says before she reaches his groin, and she stills her hand reluctantly. "This isn't going to result in my grisly death, right? Derek…"

"Knows," says a voice from the doorway and Erica jumps even though she knew he was there. Stiles nearly jumps out of his skin, leaping away from her as if she's on fire.

"Oh God, Derek, this isn't…" he's babbling.

                "Did I say I minded?" Derek asks, and Stiles looks from him to her and back again, a look of realisation slowly crossing his face.

                "Are you serious? Is this for real?" he asks, and she can see that he's actually pinching himself.

                "Only if you want to," she says softly, getting to her feet and moving slowly towards him. "Nothing you don't want, we promise."

                Derek doesn't speak and Erica has her back to him now. Stiles is looking back and forth between them again, and Erica wants to look around, see the expression on Derek's face. He smells of a mixture of nerves and lust, and she's sure she smells the same but she's pretty sure Derek needs to say something before Stiles will actually agree to this. She steps closer to Stiles, putting one hand on his shoulder and the other on his hip before she stills, waiting. Derek still doesn't speak but Stiles looks at him one last time and then steps towards her, closing the distance between them.

                Kissing him is everything she'd ever thought it would be. He's a little inept, maybe, but eager and after a moment where he doesn't seem sure what to do with his hands he gets one on her ass and starts sliding the other up under her shirt. She moans, sliding her own hand round to the back of his head so her thumb is stroking the curve of his ear and his mouth is in just the right place, even as she moves her other hand back and round under his waistband. He groans, and he smells so amazing, she wants him so much. His hand finds her breast under her shirt, teasing her nipple through the lace of her bra and making them both gasp. She pulls back slightly, breaking the kiss, to pull her shirt over her head. His pupils are blown, his lips are red and she leans in to kiss him again even as she pushes his shirt down over his shoulders. He's pretty built – nothing like Derek, obviously, but he feels amazing under her hands and she breaks the kiss again to get his undershirt off. Once it's off and thrown across the room somewhere, she starts backing him towards the bed. He's fumbling with her bra, and she's about to deal with it herself when she feels Derek step up behind her and his hands join Stiles's on the fastener. She wonders whether Stiles has actually forgotten Derek is there because he startles, hands stilling as he looks at Derek. Derek unhooks her bra no problem, then steps right up behind her so they're pressed together all the way down and Stiles's hands are trapped between them. Derek hooks his head in over her shoulder and slides his hands down to her hips. They're so close together that his knuckles are brushing Stiles's stomach, not quite his cock, and she pulls Stiles in for more kissing. Derek has removed his own shirt at some point, but not his jeans – Erica can feel his nipples and his denim-covered cock hard against her. Derek's hands are moving on her front, sliding across to the front of Stiles's jeans and Erica rocks back against Derek under the pretext of giving him more room to work. Derek shudders but still gets the button undone. Stiles has his hands on her ass, and her movement has got them lined up so Stiles can feel Derek's cock against his fingers. Stiles is biting his lip and staring as Derek unceremoniously shoves his jeans and boxers down, leaving him exposed.

                Erica and Derek inhale sharply and Stiles flushes.

                "You smell amazing," Erica says sincerely, and Stiles looks up to meet her eyes.

                "No talk of smelling," he says firmly. "I'm not there yet, it seems creepy and weird."

                "You're creepy and weird," Derek tells him but his voice has no bite to it at all. "Get on the bed."

                Stiles does, toeing off shoes, socks and ridding himself of his jeans as he goes. Erica's seen more graceful things, but Derek's unfastened her skirt, pushed it down over her hips and slipped a finger into her panties so even if she had been inclined to laugh, she's got other things on her mind. She doesn't think she's ever been this wet, and Derek's touch is making her realise just how close to the edge she is already.

                Stiles is lying on his back, eyes fixed on Derek's hand. He's reaching down to touch his own cock where it lies flat and leaking against his belly.

                "No," Erica says, more sharply than she intended. Derek's finger stills for a second and Stiles stops in his tracks.

                "Can I?" she asks, already moving forward and reaching for Stiles's cock.

                "Whatever you like," he tells her, breathless, and she gives it a slow couple of strokes to feel the long hot weight of it in her palm. He's clearly close and she squeezes the base, getting a protesting little moan for her trouble. Derek has moved over to the dresser drawer and is already holding out a condom. Erica takes it and opens it as Stiles pants, lips moving soundlessly as he counts back from twenty. Erica shucks off her panties, puts the condom on him and moves to straddle him.

                "This really isn't going to last long," Stiles warns her through gritted teeth as she takes him in hand again to guide him inside her.

                "I don't care," she tells him and it's the honest truth because all that matters right now is that she finally, finally gets him inside her. She sinks down onto him and they both groan.

                "Jesus fucking Christ, oh GOD," gasps Stiles, and then he's moving and Erica's moving too and there's no rhythm to it at all but his scent is everywhere and she can feel him coming inside her, hands scrabbling at her sides as he slams up into her, and she's coming too, clenching around him and moaning like she's the one who's never done this before.

                Derek's been standing by the bed, naked and touching himself, and before their breathing stills he's putting on a condom and moving onto the bed behind her, nudging Stiles's legs apart to make space. She moves slightly, sliding her hand down to hold the condom on as she eases Stiles out of her. He kisses her as she moves, reaching down himself to take the condom off. Derek isn't waiting, he's already lined himself up and Erica leans down so her breasts are pressing against Stiles's chest, weight on her elbows as Derek thrusts into her with a grunt. He holds steady for a second and he and Stiles are staring at each other again. When Derek starts moving, Stiles seems to take that as permission. His fingers circle her clit, sending shocks through her in the wake of her previous orgasm, and then slide down to find her entrance, one finger either side of Derek's cock as he moves in and out of her. The palm of Stiles's hand is in just the right place for her to grind against it and she does so, setting up a pace that she knows will drive Derek wild. Stiles wriggles under her, trying to free his other hand. He's hard again already, she can feel it, but he's reaching up to touch Derek and she can feel that Derek is right on the edge, thrusts getting faster and more desperate. Stiles puts his hand on Derek's arm and Derek shudders, hips snapping forward another couple of times as he comes silently.

                The bed isn't really big enough for three. It's a double, bought in case Erica had a fit in the night, but when one of you is Derek Hale, any bed other than king size is going to feel too small. After Derek pulls out of her, wrapping up both his condom and the one Stiles had just abandoned on the bed, Erica positioned herself against Stiles's back. Derek hesitates before joining them, and it's Stiles who speaks.

                "Come on, man, I know you're big but you're not _that_ big, there's room."

                Erica snorts.

                "Oh my God, not like that, for the love of God!" he babbles and literally slaps a hand over his mouth. Derek smirks and settles himself on the bed facing them, propping his head on his hand. He looks relaxed but he's not touching Stiles at all. Erica shifts slightly, throwing her leg over Stiles to hook one of Derek's ankles and draw him in. Stiles, never overawed for long, removes his hand from his mouth. His eyes are fixed on Derek as he reaches out, cautiously but with clear intent, to place his hand on Derek's chest. Both Erica and Derek inhale sharply.

                "Again with the smelling thing," says Stiles. "Creepy, weird and a majorly unfair advantage."

                "All I can smell right now is sex," Derek admits and Stiles slides his hand up to the nape of Derek's neck and leans in to kiss him. Derek shifts closer, so he's not quite so near to the edge of the bed, and gets a leg between Stiles's so the other guy can rub up against him. Erica watches for a moment or so before starting to suck kisses along Stiles's shoulder blades where the bone moves under the skin, and down along the line of his back to the jut of his hip. She licks around Derek's fingers where they're kneading Stiles's ass, and that gets groans from both of them. Then she reaches over to the still-open drawer to snag two more condoms and the bottle of lube. When she slides a slick finger between Stiles's buttocks to just rest on his hole, he freezes. Erica and Derek freeze too, though Erica doesn't move her hand. Derek pulls back slightly to look at Stiles's face.

                "I'm not sure about this," Stiles says. Erica expects him to keep speaking, to tell them exactly what he's thinking, but he doesn't say anything else and she's forced to move so she can see his face. Her finger slips as she does so and he tenses again.

                "Sorry," she says, and looks at Derek for guidance.

                "It's either you in the middle or her," he says, and Erica makes a desperate little noise before she can stop herself.

                "Wow, really?" Stiles says. "It'd be that good?" He twists slightly to get a better view of her face, and this time when her finger moves between his legs he doesn't tense. His pupils are wide, his lips red – he looks totally debauched and Erica knows she must look the same.


	2. In which Stiles is in the middle and Erica is in charge

"You'll be gentle with me?" Stiles asks, and Erica moves her finger a little tiny bit, watches his eyelids flutter.

                "Yes," Derek says in a low voice. He leans forward to put his mouth on Stiles's chest, kissing across it until he reaches a nipple and closes his mouth around it. Erica takes her hand away and presses Stiles's hip so he's flat on his back. His cock is thick, resting against his belly, and she reaches out to touch it, wrapping a hand around it and stroking gently. She wants to memorise the way he looks, the feel of his skin under her hands, his taste…

                She leans forward and licks a stripe up his cock, making him gasp. He tastes of salt and latex and come; Erica curls her tongue around the tip as she eases her finger inside him.

                "Oh Jesus Christ," Stiles says, and Derek moves up to kiss him. Stiles kisses back roughly but Derek gentles him. Erica wraps her lips around Stiles's cock and pushes another finger inside him, crooking them to find his sweet spot, and it's a good job that Derek is practically lying on him because Stiles bucks up so hard they'd probably have both been off the bed. As it is, Erica has to pull back fast but she's a werewolf so he doesn't hurt her, even if he does make her eyes water. Derek stops kissing Stiles to look down at her, a slight hint of amusement in his eyes that fades when he focuses on where her lips are still wrapped around Stiles's cock.

                "Condom," he says sharply and both Erica and Stiles make noises of distress.

                "Erica," Derek says warningly, and she pulls off as slowly as she can, licking her lips. Her fingers are still moving in and out of Stiles and she doesn't let them slow.

                "I'll let you do the condom," she tells Derek sweetly and Stiles groans. Derek reaches for one of the condoms on the bed, opening the little packet with hands that just might be shaking a bit before reaching for Stiles's cock. Stiles props himself up on his elbows to see better and Erica takes the chance to add more lube and a third finger, loving the way his body stretches around her.  Stiles makes a little choked-off whimpering noise as Derek wraps his hand around his cock. Erica watches as he strokes it more tentatively than she'd have expected, a learning stroke. Derek's eyes are fixed on his hand and he looks…Erica has no words for how he looks. She times her fingers to his stroke and Stiles makes a desperate sound. Derek squeezes his cock hard around the base despite Stiles's protest, and then rolls the condom on.

                "Move," he says to Erica, and she does, but reluctantly, and Stiles sighs when her fingers slip out of him. Derek slides his own in almost as soon as hers are out, and Erica thinks it's more by accident than design that his fingers hit the sweet spot that has Stiles arching off the bed.

                "I thought you said you were going to be gentle," he gasps and he looks totally wrecked. Derek gives him a smile that has too many teeth in it to be truly reassuring.

                "We'll look after you," he says, and Stiles actually relaxes a little. Erica wipes the lube off her hand on a tissue and then slides her fingers between her own legs to work her clit. Stiles is watching her and Derek pulls back, reaching for the second condom.

                "Can I?" Stiles asks, and Derek rocks back on his haunches.

                "Go ahead," he says. Stiles kneels up too and kisses him gently, just a peck on the lips, before reaching for the condom. He fumbles a bit opening it; he flushes but neither of them move to help him and he gets it open on the second try. He gives Derek's cock a couple of strokes and Derek growls slightly, a low rumble deep in his chest.

                "Easy there, big guy," Stiles says, rolling the condom on cautiously. Derek leans forward and kisses him again, a proper deep kiss and it's Stiles who breaks away first. Erica moves to kiss him and he pushes her back onto the bed, sliding inside her with much more confidence the second time around. She grins up at him, a fierce, affectionate grin, and rocks up to meet him. They find a rhythm in a couple of strokes and then Derek is shoving a pillow under her hips and sliding a hand up Stiles's thigh. Stiles pushes into her and then stills, buried to the hilt as Derek fits himself behind him and starts pressing forward. Stiles's eyes go wide and he makes a pained noise that Derek disregards, moving forward steadily.

                "We've got you," Erica promises, cupping Stiles's face in one hand and running the other soothingly up his side. She watches the emotions flickering across his face, listens to the way his heart races as he breathes great ragged breaths.

                "Wow," Stiles says and Derek pulls out slightly before pushing back in again and then they are all moving in a disjointed tangle of limbs that isn't quite getting anyone what they need.

                "Stop," Erica growls in frustration and she's kind of surprised when both of them actually do. Derek looks _wrecked;_ she wonders if he might come before Stiles.

                "Let Stiles drive," she says and Derek pulls back before stilling with visible effort. Stiles moves slowly back to meet him, almost slipping out of her before he rocks back forward. She rises to meet him and this time she follows him a bit as he moves back so he's still safe inside her. A few thrusts like that and Stiles begins to pick up the pace. Erica can see the concentration on his face, loves the way he's biting his lip. Derek is looking more in control now; she pinches his arm just to make sure and he growls at her.

                Stiles seems to take that as some sort of signal because he gasps and thrusts harder. Now they've found their rhythm and it's just perfect, slick wet and friction. Erica is moaning, she can't stop, and she'd feel kind of ashamed but it seems to be driving both guys wild.

                She's right – Derek does come before Stiles but mostly because she digs her claws into him when her orgasm hits unexpectedly, crashing over her like a wave, and she barely notices when Stiles begins to come.

                She notices when he flops down on top of her with all his weight though. Derek has just enough presence of mind not to add his own weight to the pile, pulling awkwardly out of Stiles before collapsing beside them with one arm over them both. Stiles turns to kiss him and Erica presses at where their mouths are joined until they let her into a messy three-way kiss. When Erica shifts slightly, they stop kissing and Siles reaches down to ease himself out of her, handing the condom to Derek (much to his disgust) before settling down in much the same place.

                "Oh come on, you're a werewolf," Stiles says when she makes a little protesting noise. "You can handle the weight," but Derek rearranges them so Stiles is sprawled across them both, and no one objects. Stiles relaxes easily, going limp against them as his heart rate slows. When he's almost asleep, Erica turns to her Alpha for reassurance. He kisses her warmly, pets her hair, and she strokes his arm and wonders for a second just who is reassuring who.

 

**Notes for the Chapter:**

> The End! Or you could read chapter 3, in which Erica gets fucked by both Derek and Stiles...


	3. In which everyone agrees that Erica has the best ideas

"Haven't tried it before, but…" Erica says, and she makes a gesture which was supposed to explain just how mind-blowingly hot taking them both at once would be. She suspects it just looks like a wave at best and a flail at worst, but Stiles just says

                "We can do that, right?"

                When Derek nods, Erica moves her hand away from Stiles's ass and slides back up their bodies to shower first Derek, then Stiles with fierce, desperate kisses.

                "Easy," Derek says softly, but he doesn't make it easy, pressing her down on the bed and sucking kisses down her neck, biting at her collarbone with one hand squeezing at her breast. She grabs Stiles's hand and guides it to her other breast. His hands are more gentle than Derek's, his touch more curious, and the contrast makes her moan. Derek is moving lower now, leaving a trail of red marks across her belly and on her inner thighs before he puts his mouth where she wants it.

                "Oh, that is so hot," Stiles breathes, and Erica gasps as Derek licks down from her clit to her ass, circling her hole with his tongue. She flails for the lube but he's probing and he's had his tongue inside her cunt before but she's not sure she can relax enough to let him put his tongue in her ass. When he licks back up and pushes his tongue inside her cunt instead, she consciously relaxes against his fingers, enough to let him slide a finger inside her. He works his tongue and his fingers together to massage the skin between them and oh my god, this might be the best idea she's ever had. She makes a protesting little sound when Derek pulls his tongue out of her, but it changes to a groan as he goes back to her clit.

                "Good girl," Derek says, low against her clit, and then he decides to stop messing around and starts licking her in earnest, thumb in her cunt and fingers working inside her ass, twisting and stretching her, preparing her for the most debauched thing she's ever done.

                Stiles is moving now, teasing at her breasts with his lips and his teeth, hands roaming over her skin.  Her whole body is on fire, she's so close now she knows one more touch, one more lick will send her over the edge. She's right…

                "My turn?" says Stiles, and he probably didn't mean it to come out as a question but there's just a tiny tremble to his voice that would let Derek refuse if he liked. Instead, Derek pulls away from her and Erica swears at the loss of sensation.

                "Go ahead," Derek says with a smirk, the bastard, and then he's moving away, wiping his fingers on a tissue. Stiles looks at her, just looks, and Erica can feel herself blushing.

                "Just touch me!" she snaps, and Stiles's lips quirk and he runs a finger around her clit, making her gasp.

                "Right there," she says urgently, as if he hasn't worked it out, but he's already sliding his finger lower, to where her cunt is slick from Derek's tongue. He slips a curious finger inside her, he's definitely sussing out the territory rather than working with an aim in mind and this is torture. She squeezes down on him, hard, as Derek says:

                "She can take more than that."

                "This is so filthy," Stiles says. "I cannot believe how filthy this is," but he's sliding two fingers into her cunt now and moving them like he means business and Erica moans in encouragement.

                Derek moves to kiss Stiles, but Stiles pulls away.

                "You had your tongue in her ass," he says, sounding a combination of appalled and really turned on.

                "I had my tongue in her cunt as well," Derek point out, and Erica has a brief moment of concern about that before deciding that her werewolf state will probably make any infection short-lived and deciding not to stress.

                "Yeah, you did," Stiles says thoughtfully and then he's leaning down to put his mouth on her clit.

                "Oh FUCK, yeah," she babbles. "Stiles!"

                She has to struggle not to buck up against him, but after the first tentative couple of licks he presses a little harder and she breaks apart underneath him, gasping his name.

                When he sits up he's looking smug, but Erica's too boneless to care. Derek moves to kiss him and Stiles has either forgotten about her ass or decided he doesn't care because he's kissing back desperately, his free hand scrabbling at Derek's shoulder. Derek pushes him back onto the bed, forcing Erica to shift slightly so she doesn't get crushed and grumbling slightly as she loses the pressure of Stiles's fingers inside her. Derek gets Stiles underneath him and Stiles immediately wraps his thighs around Derek's waist and his hands onto Derek's ass, pulling their bodies together and rubbing up, getting a growl from Derek. Erica watches, trying to memorise every detail because this has got to be the hottest thing she's ever seen.

                She can't resist touching for long though, deliberately hitting all of Derek's sweet spots. Stiles is a quick learner; after she's got one or two deep moans out of Derek he rearranges his hands and touched until Derek pulls away sharply, breathing heavily.

                "Enough!" he snarls as Stiles moves to follow. He's sitting back on his haunches, biting his lip, hands pressed against his own thighs.

                "Whoa, seriously?" Stiles says and Derek avoids their eyes. Stiles gives Erica a triumphant grin that she can't help returning.

                "Erica," Derek says, note of command in his voice, and Erica moves to take his place above Stiles, rolling a condom on before sliding down on him. He grins up at her and slips one hand into her hair, pulling her down to kiss him even as he thrusts up into her. Erica moans into his mouth as he kisses her deeply and enthusiastically. He tastes a little of Derek, and she simultaneously wants to get more of that taste and to eliminate it completely. She feels Derek's hand on her back, pressing her down so she's flush against Stiles before he strokes down her side to the curve of her ass before circling her hole. He's clearly not in the mood for waiting, she relaxes as best she can as he shoves two fingers inside her.

                "Oh my God, I can feel _everything_ ," Stiles says, eyes wide and shocked as Derek works his fingers in and out of her. Erica's sure she looks just as shocked but she's not really doing the words thing any more, trying to shove back onto Derek and grind down onto Stiles at the same time. Derek puts in a third finger and all three of them groan.    

                "For the love of God, fuck her already!" Stiles gasps and Erica kisses all over his face in gratitude as Derek finally, finally slides inside her. He takes it slow, stops when he's fully inside her to let her adjust. He's flush against her back, and she can feel him shaking. She tries to move, to draw herself up so she can slide herself back down, but the angle's not right, maybe they should have put a pillow under Stiles's ass or something because she can't get purchase. Stiles puts a hand on her hip and she stills and then he's rocking his hips down and away from her to thrust back in, maybe only an inch or so but it feels almost too much. Derek growls and he's moving away, pulling out and both she and Stiles groan as everything suddenly gets looser, emptier. Stiles rocks up hard and Derek slides back in at almost the same time and it's just…wow.

                "God, that's amazing," Stiles babbles. "Holy hell, do that again, " and Derek obliges, faster this time and then somehow they've got a rhythm going, smooth and sweet and all Erica can do is lie there and take what they give her. She can feel them both all around her, cocks and fingers and tongues and skin and the overwhelming scent of _lust_ whenever she breathes in. She shifts ever so slightly the next time Derek pulls out and when he thrusts back in she almost bucks off the bed. Stiles is talking constantly, a stream of "Oh God, fuck, yeah, please," that is driving her crazy in the good way. He also has his hands between her and Derek, pinching at one of Derek's nipples even as his other hand smooths down her back to touch where Derek is inside her, and then past to touch where he is inside her. Erica clenches, making them both moan. She can feel when Derek loses it, his thrusts speeding up and this is right, it's perfect. He's growling and she's pretty sure she's growling too as she clenches again, feeling rushing through her body so she swears she can feel her orgasm even in the tips of her fingers. Stiles is crying out underneath her, clutching her ass so hard he'd definitely leave marks if she were human, and gradually they stop moving.

                Derek pulls out of her cautiously and Stiles reluctantly does the same. Derek strips off his condom and hands it to Stiles.

                "You can deal with those this time," Derek says and Stiles makes a complaining noise but does as he's told before slotting himself back on the bed, curled against her back. Derek immediately moves so his arm is around both of them and Erica enjoys the feeling of being snuggled and warm.

                "We should do this again," says Stiles dreamily, and Derek makes a noise that might or might not be agreement. Erica's half-asleep herself, and she licks at Derek's chest, tasting sweat and musk.

                "It was a good idea," he agrees, but she thinks Stiles is already asleep.

 

** The End. **

**Notes for the Chapter:**

> Well, if you read all three chapters, then thank you! If you skipped chapter 2, you missed a Stiles sandwich... Hope you enjoyed it :-)

**Author's Note:**

> SO. I accidentally wrote choose your own adventure porn! For the ending where Stiles is in the middle but Erica is in charge, read Chapter 2. For the ending where Erica is in the middle and Stiles is talkative, read chapter 3. If you just like threesome porn with a hint of OT3, read both :-)


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